NOx & SCR diagnostic workflow
SCR effectiveness depends on exhaust temperature, reductant quality and delivery, NOx measurement, catalyst condition and correct engine-out emissions. Replacing one sensor or suppressing one code does not diagnose the system.
Controlled from identification to delivery.
Software must not be used to bypass required SCR, OBD or inducement functions on public-road vehicles.
Treat SCR as a chain
Review upstream and downstream NOx values, temperature, dosing command, reductant pressure and quality, crystallization, leaks and catalyst conditions. Engine faults can also raise engine-out NOx.
Validate plausibility
A NOx sensor is a networked electronic module on many platforms. Check supply, ground, communication, warm-up and plausibility against operating state before replacement.
Legal boundary
SCR and inducement systems are regulated emissions functions. Public-road vehicles must be repaired and remain compliant with the law that applies to them.
Research trail.
Clean Air Act prohibits defeat devicesUnited States Environmental Protection Agency · accessed 01 Sep 2026Before you submit.
Does a P20EE code always mean the SCR catalyst is bad?+
No. Dosing, temperature, NOx sensing, exhaust leaks, reductant quality and engine-out emissions must be evaluated before condemning the catalyst.